1. Field of the Invention
The present invention relates to the improvement of an exposure control means in a camera having a centralized control system that has a built-in control circuit such as a microprocessor etc. and employing a shutter wherein an optimum exposure is obtained in a way that the exposure is given to the film between the gradual opening of the shutter and the closing thereof and the correlations between the shutter apertures and their corresponding lapse of time form a triangle on the graph. It further relates to the improvement of the exposure control means for flash photographing in a camera employing aforesaid shutter and built-in strobe flash light source.
2. Description of the Prior Art
With the spread of electronics used in a camera, various functions of a camera have been diversified with their performance improved. While the shutter that is a basis of an exposure control is still continuing its conventional construction and no remarkable progress has been observed in particular. The majority of the program control shutter which is a shutter-cum-diaphragm for the present small-sized camera is the one wherein the gradual opening of the shutter gives an exposure to the film and the closing thereof gives an optimum exposure and the correlations between the shutter apertures and their corresponding lapse of time form a triangle on the graph The shutters of such type are disclosed in Japanese Patent Examined Publication Nos. 18111/1971 and 11947/1969 for example.
In the technology of aforesaid shutter, the capacitor is charged with photoelectric current from the light-responsive element from the moment of the start of shutter opening and then the shutter closing is performed when the voltage charged in the capacitor reaches a certain level of voltage, namely, when the optimum exposure is given to the film. This system is publicly known and therefore the detailed description thereof will not be made here but it has demerits on the points of characteristics of light-responsive element and complicated mechanical structure of the shutter, as itemized below.
(1) When CdS is used as a light-responsive element, the .gamma. value that represents the relation between the quantity of light and the resistance value for CdS needs to be 0.5.
(2) There is a limitation for the opening speed of the shutter.
Despite the spread of electronics in a camera mentioned above, the shutter itself still consists obstinately of mechanical factors which have prevented the simplification of camera structure.
On the other hand, an aperture value F that gives an optimum exposure in flash photographing is obtained, as is generally known, from the equation of F=GNO/L, where "GNO" is a guide number of a strobe flash and "L" is a subject distance. Following two ways have been known as an application of the equation.
(1) An aperture is to be changed according to the subject distance. Namely, it is a method of so-called flash-matic mechanism wherein an aperture diameter is changed being interlocked with the turning of a focusing ring.
(2) A method that employs a shutter-cum-stop and causes a strobe flash to emit light during the course of gradual opening of the shutter ranging from the closing to the opening thereof and changes its emission timing according to the subject distance.
The former method makes it very difficult to photograph using a DX film and it is actually impossible to incorporate into a small-sized camera the flash-matic mechanism that makes the photographing on the DX film possible, which is a demerit. The mechanism that detects the ISO speed from the DX film and changes the stop value through the mechanical driving thereof using the electric signal converted from the ISO speed detected requires electro-magnetic devices such as a motor, a magnet and others as a driving source and thereby it is actually difficult to incorporate such mechanism into a small-sized camera.
Since the control of emission timing for strobe flash is made electrically in the method of (2), it is easy to introduce the information of ISO speed obtained from a DX film into the control system and therefore the method (2) is suitable for the use of DX films.
However, the following problem is caused by the application of the method (2) to a camera. An emission timing of the strobe flash may be obtained through the calculation that employs the aforesaid equation and the correlation between the shutter aperture and time shown in FIG. 1. Heretofore, the calculation has been performed for the determination of the emission timing of the strobe flash under the assumption that the shutter aperture changes linearly and ideally with the lapse of time as shown in FIG. 1. However, it is impossible for the actual shutter to have such an ideal aperture characteristic. It is known that the serious nonlinearity in the aperture characteristic is caused by the type of the shutter mechanism employed or the like and it has become clear that the emission timing obtained from the simple calculation like the one explained above can not provide an optimum exposure due to the aforesaid nonlinearity.